Mixers are used in communications circuits for the purpose of generating a modulated carrier for transmission, demodulating a modulated carrier in reception, or converting a signal at some input intermediate frequency (IF) to another output radio frequency (RF) by multiplying two input signals and thereby generating a third. A number of mixer realizations, both passive and active, are known in the art, and double-balanced mixers are known particularly well due to their advantages in the suppression of unwanted spurious signals and the isolation of any one of three ports to the other two, there generally being two inputs and one output. The Gilbert Cell has been the most widely used active mixer circuit for performing the above tasks, usually incorporated within an integrated circuit. It does, however, possess certain limitations in terms of intermodulation distortion that precludes it's use in communications systems having demanding performance specifications.
Referring to FIG. 1, a schematic diagram of a Gilbert cell mixer is shown in one of many known topologies. Here, the mixer is comprised of transistors 101, 102, 104, and 105, which are turned on (saturation) and off (cutoff) alternately by a differentially applied local oscillator (LO) signal. By this switching action, a pair of currents generated by transistors 103 and 106 are divided into four paths, there being two paths for each of two currents. The currents generated by transistors 103 and 106 are the result of an input intermediate frequency (IF) signal applied differentially across their respective base connections and the emitter resistors 109 and 110. The current source I11 serves to establish the quiescent bias condition of the mixer, and the resistors 109 and 110, in conduction with the incremental emitter resistances r.sub.e of transistors 103 and 106, and the load resistors 107 and 108, serve to establish the conversion gain. Those familiar with the art readily understand the signal-handling limitations of the Gilbert cell, the performance variations over temperature due to the temperature dependency of r.sub.e, and the other tradeoffs necessary in order to meet system design requirements of noise, intermodulation (IM), power consumption, and gain stability over temperature. It has long been desireable that a Gilbert cell mixer be available that has improved IM and temperature performance without the expense of added power.
In an ettempt to reduce the intermodulation distortion of the Gilbert Cell mixer, Heinz et al, in U.S. Pat. No. 5,410,744 entitled "HF Mixer Stage having a Common Base Circuit", applied the input baseband signal to the driver transistors (items T1 and T2) at their emitter terminals, a method known to the art as common-base, thus increasing the large-signal behavior and therefore the dynamic range and intermodulation performance. This method does not, however, account for the nonlinear characteristics of the incremental emitter resistance r.sub.e of the driver transistors, a significant source of intermodulation distortion, nor does it account for the nonlinearities of the four switching transistors (items T3, T4, T5, and T6). Furthermore, this method does not address the temperature dependency of the incremental emitter resistance r.sub.e, therefore offering little or no improvement in the temperature performance.
In a further attempt to reduce the intermodulation distortion of the Gilbert Cell mixer, Heck, in U.S. Pat. No. 5,548,840 entitled "Balanced Mixer Circuit with Improved Linearity", describes a Gilbert Cell mixer to which has been added a pair of transconductance amplifiers, each of which is used to ensure that the current passing through the resistors R1 (items 322 and 324) is a faithful reproduction of the applied input IF signal. The method does not, however, account for the nonlinearities of the small-signal gain of the driver transistors (items 314 and 316), nor still does it account for the nonlinearities of the four switching transistors (items 306, 308, 310, and 312). It does, however, offer a considerable advantage in temperature performance over other methods.
In his paper "The SL6400 High Performance Integrated Circuit Mixer," Peter Chadwick properly recognizes that the intermodulation in the Gilbert Cell mixer (referred to by Chadwick as a transistor tree mixer) is caused by the nonlinearity of the voltage to current conversion in the driver (signal input) transistors. He then describes a Gilbert Cell mixer in which the driver transistors have been replaced with linearizing amplifiers, each consisting of an NPN and a PNP transistor. This method provides a good improvements in linearity and temperature, but at the expense of considerable power consumption.
It is the purpose of this invention to address the above sources of distortion and temperature variation, and to therefore provide an active mixer of markedly improved performance, while at the same time conserving power consumption and retaining an overall sense of simplicity and cost effectiveness.